LEDs have the features of low power consumption and extremely high luminous efficiency, and in recent years have come to be used in various fields such as display devices, illumination devices, and advertisement devices.
The luminance of such LEDs is generally controlled using a pulse width modulation scheme.
When the luminance of LEDs is controlled with a pulse width modulation scheme (PWM scheme) in this manner, a PWM signal is supplied to an LED driver and the LED driver adjusts the luminance of the LEDs on the basis of this PWM signal.
The LEDs turn off when the PWM signal is low and the LEDs turn on when the PWM signal is high, and the luminance of the LEDs is adjusted according to a low/high period ratio in the PWM signal (on/off duty of the PWM signal).
However, when the LED driver is activated and the PWM signal enters a high state from a low state, an excess current flows in a power source and a defect such as the melting of a fuse mounted in a module is known to occur.
Therefore, Patent Document 1 discloses a configuration provided with a soft start function (function that prevents an excess current from flowing in an LED) that is driven when a PWM signal enters a high state from a low state when an LED driver is activated (after an enable signal has entered a high state from a low state).
FIG. 7 is a drawing showing the start timing and cancellation timing of the soft start function in an LED driving device provided with the soft start function disclosed in the aforementioned Patent Document 1.
FIG. 7(a) shows the case where a high state period of a PWM signal is set to be longer than a low state period, and where the soft start function turns on at the same time that the PWM signal enters a high state from a low state when the LED driver is activated. The soft start function then turns off and is completely canceled when a period that is approximately half of the high state period of the PWM signal has elapsed.
FIG. 7(b) shows the case where the high state period of a PWM signal is set to be shorter than the low state period, and where the soft start function turns on at the same time that the PWM signal enters a high state from a low state when the LED driver is activated, but it is difficult to turn the soft start function off during the high state period of the PWM signal as in FIG. 7(a) because the high state period of the PWM signal is short.
FIG. 7(c) shows the case where, in a situation such as that of FIG. 7(b), the number of times that a PWM signal rises is counted and, at the third signal, the soft start function is turned off and the soft start function is completely canceled.
As described above, the aforementioned Patent Document 1 discloses a configuration in which the elapsed time from a PWM signal entering a high state when an LED driver is activated is counted, and when that count value has reached a prescribed value, the soft start function thereafter is disabled. The aforementioned Patent Document 1 also describes that it becomes possible to greatly reduce the on duty of the PWM signal by using such a configuration, and as a result it is possible to realize an LED driver circuit that can control a load such as an LED to a low driving range with high precision.